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Register a new userEvidence for a narrow structure at W~1.68 GeV in eta photoproduction on the neutron
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New results on quasi-free $eta$ photoproduction on the neutron and proton bound in a deuteron target are presented. The $gamma n to eta n$ quasi-free cross section reveals a bump-like structure which is not seen in the cross section on the proton. This structure may signal the existence of a relatively narrow ($Msim 1.68$ GeV, $Gamma leq 30$ MeV) baryon state.
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New (preliminary) data on eta photoproduction off the neutron are presented. These data reveal a resonant structure at W=1.67 GeV.
First measurement of the beam asymmetry $Sigma$ for Compton scattering off the proton in the energy range $E_{gamma}=0.85 - 1.25$ GeV is presented. The data reveals two narrow structures at $E_{gamma}= 1.036$ and $E_{gamma}=1.119$ GeV. They may signal narrow resonances with masses near $1.68$ and $1.72$ GeV, or they may be generated by the sub-threshold $KLambda$ and $omega p$ production. Their decisive identification requires additional theoretical and experimental efforts.
A narrow dip structure has been observed at 1.9 GeV/c$^2$ in a study of diffractive photoproduction of the $~3pi^+3pi^-$ final state performed by the Fermilab experiment E687.
Revised analysis of $Sigma$ beam asymmetry for $eta$ photoproduction on the free proton reveals a resonant structure at $Wsim 1.69$ GeV. Comparison of experimental data with multipole decomposition based on the E429 solution of the SAID partial wave analysis and including narrow states, suggests a narrow ($Gamma leq 15$ MeV) resonance. Possible candidates are $P_{11}$, $P_{13}$, or $D_{13}$ resonances. The result is considered in conjunction with the recent evidence for a bump-like structure at $Wsim 1.67 - 1.68$ GeV in quasi-free $eta$ photoproduction on the neutron.
The photoproduction of $eta$-mesons off nucleons bound in $^2$H and $^3$He has been measured in coincidence with recoil protons and recoil neutrons for incident photon energies from threshold up to 1.4 GeV. The experiments were performed at the Mainz MAMI accelerator, using the Glasgow tagged photon facility. Decay photons from the $etarightarrow 2gamma$ and $etarightarrow 3pi^0$ decays and the recoil nucleons were detected with an almost $4pi$ electromagnetic calorimeter combining the Crystal Ball and TAPS detectors. The data from both targets are of excellent statistical quality and show a narrow structure in the excitation function of $gamma nrightarrow neta$. The results from the two measurements are consistent taking into account the expected effects from nuclear Fermi motion. The best estimates for position and intrinsic width of the structure are $W$ = (1670$pm$5) MeV and $Gamma$ =(30$pm$15) MeV. For the first time precise results for the angular dependence of this structure have been extracted.
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